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We’ve had an old Bakelite phone for years as nothing more than an ornament, because some of its innards are missing.
I’d been itching to hide something in it for quite some time, but I was loathe to damage the Bakelite casing.

I removed all the parts I could, so that I could use the threaded holes and screws for mounting the new 21st Century innards!

The hook-switch contacts were carboned up, so I cleaned them with some P1600 abrasive paper, and tested them with a multimeter.

Rather than using just the hook-switch to trigger the Voice HAT, I decided to use the dial as well, to avoid false triggering if the receiver gets knocked.
I wired the switches in series, in a logic AND configuration; this requires the handset to be lifted and dial to be rotated to trigger.

Again with the steel base-plate, I really didn’t want to drill it to mount the Raspberry Pi. Were it not for the circuit diagram printed onto a large paper label and stuck to the base, I’d have used self-adhesive PCB mounts. Instead I opted for magnets!!

Because I couldn’t find enough technical data on the Voice HAT, and I wasn’t sure how the pulsing LED was powered, I used an opto-isolator to trigger a pair of transistors. One transistor powers each strip’s blue LEDs, whereas the green LEDs are connected straight to the power so that they are on constantly whilst powered up.

The board containing the opto-isolator and transistors is also held in place with magnets!

Colour-check quality approval.

The speaker included with the AIY kit is waaayyyyy too big to fit in here, so I bought a 4W subminiature speaker instead. I crafted a horn from some 3mm ABS and a Bosch glue-gun, to channel the sound through the little holes — where the sound from the bells would have originally escaped!

The sound does unfortunately sound “plasticky” — plywood would likely be better for this — maybe a future modification…

To avoid voltage drop in the supply, I opted to use 12V power in, then a DC-DC converter. The converter I bought states it can handle 15W and comes with USB A sockets on flying leads. I was able to mount the module using existing holes and non-metric screws; you can see the module in the top-right of the next image.

The original cable is fabric-covered, and it is possible to buy new mains cable in this style. However, I’m also running an ethernet cable out of the back so I used expandable nylon braided sleeving to cover the flat ethernet cable and power cable. I underestimated just how much this stuff can expand to accommodate plugs passing through (like a snake swallowing a meal!) so the current sleeving is way over-sized!

So, future mods include improving the speaker enclosure, and replacing the cable sleeve with some of narrower diameter.

My brother-in-law mentioned that he needed a thermal switch for the fan in his shed. The building works as a heat-trap during the day, and he’d like a way of blowing the hot air out, so that it’s more habitable when he gets home from work.

I have an abundance of Picaxe chips, so that’s what I decided to use for this project, specifically the 20M2 variant. I also used a second chip to make a serialised LCD: This means the main chip only needs one data line to operate the LCD. It also made testing & development easier for me.
I could have used a RaspberryPi Zero, but I’m still at beginner level with Python, so this would have ended up as a winter-heating project instead!!

The main programme measures the ambient temperature and compares this to the user-set temperature: if equal to or above, then the fan switches on. Hysteresis of 2°C is written in to the programme, which may need altering, but testing needs to be done.

The temperature probe is a DS18B20, and support for these devices is written in to the PICAXE firmware. This makes things easy!

A hole can be drilled for access to the programming socket, but it may not need reprogramming…

I’ve made this project modular for two reasons: 1) I only have short periods of tinkering time, and can easily forget where I got up to with large projects, and 2) it aids fault-finding.
A third benefit is being able to use manufactured modules / kits instead of Veroboard. E.g. the relay module kit cost about the same as a relay on its own!

When the user changes the trigger temperature, it is stored in EEPROM in case the power is turned off. The value is loaded upon power up, with the assumption that the user will have an optimum temperature.

As this is running from a solar installation, it makes sense to monitor the voltage of the lead-acid battery, AND have a cut-off should the voltage fall too low.
Because this is a 5V system; a voltage-divider is required to measure higher voltages, so that the chip isn’t fried by receiving a voltage higher than its supply voltage. This makes use of the established potential divider equation:

V_out = (R2 / R1 + R2 ) • V_in

There is also a piezeo disc that will sound a warning if the battery voltage is too low.

The yellow button in the middle changes the mode of the unit from ‘Auto’ to ‘on’ to ‘off’.